Jet fuel often becomes contaminated in a fuel tank of a turbine engine aircraft with small quantities of free water from condensation arising from the changes in temperature due to altitude changes. On the ground the fuel/tank temperature can range from −18° C. to +40° C., whilst in flight it typically ranges from −22° C. to −39° C.
Over a number of temperature change cycles, for example over a number flights, condensation of the water vapour can give rise to the accumulation of water within the fuel tank which may exist as a separate phase or free water within the fuel. If the free water is permitted to pool and freeze in the fuel tank, it can form slugs of ice (ice particles of sufficient size such that they may be trapped in the fuel filtering system) which can be potentially harmful to the function of the aircraft engines. Indeed, it is believed a Boeing 777 aircraft lost sufficient power to cause an emergency landing at Heathrow in January 2008 due to the formation of ice reducing the flow of fuel from the fuel tanks to the engines (AAIB interim report No 2 G-YMMM).
At present, as an alternative to employing fuel tank heaters, materials such as diethylene glycol monomethyl ether (DiEGME) are mixed with aircraft fuel to prevent ice formation in the fuel. Whilst DiEGME is about equally miscible in both water and fuel at temperatures above freezing, careful monitoring during the mixing process must be adhered to at all times to ensure an initial homogenous fuel. However, no matter how carefully mixed, DiEGME has a tendency at temperatures significantly below freezing to preferentially concentrate in the water phase. Thus, due to disproportionate distribution of DiEGME in the water and fuel at low temperatures, insufficient DiEGME in the fuel phase can lead to the formation of a separate aqueous phase (water and DiEGME) in the fuel. The presence of the DiEGME in the aqueous phase will prevent some of the water in this phase from turning to ice. However, the DiEGME/water mixture has an unusual characteristic in that it forms a gel like substance at low temperatures: the gel like substance is commonly referred to as “apple jelly” in the aviation industry. The US Federal Aviation Authority has attributed several aviation accidents to the formation of this “apple jelly” material in aircraft fuel tanks.
It is an object of the present invention to reduce or eliminate the formation of ice slugs and apple jelly in fuel in the fuel tanks of turbine engine aircraft.
The use of water as an additive in fuel oils to reduce emissions of pollutants and to aid incorporation of other beneficial performance additives has been known for many years. The use of water as an additive in lubricant oils to improve the cooling properties of e.g. cutting oils has also been known for many years. Water is incorporated into the fuel and lubricant oils in the form of a water-in-oil emulsion.
Water-in-oil emulsions formed with a large water droplet size tend to have a milky appearance. These emulsions require a number of secondary additives such as corrosion inhibitors and bactericides to overcome problems associated with addition of the water phase. These macroemulsions, due to their large water droplet size, also tend to exhibit instability that leads to oil/water separation. Naturally, this is unwelcome as it may lead to problems with not only machine failure but also problems with ignition e.g. in a diesel-engine.
Cutting oils, based on water-in-oil emulsions, have been used to lubricate machine tools. The excellent coolant property of the water has been demonstrated to improve the life of the tool. However, the incorporation of water coupled with the instability of macroemulsions give rise to other problems, such as the lubricity of the oil, which is decreased with addition of water thereby affecting the surface finish of the metal.
Water-in-oil emulsions formed with an average water droplet size of 0.25 μm or less, preferably of 0.1 μm or less, more preferably of from 0.03 μm to 0.08 μm (hereafter referred to as “microemulsions”) are translucent. A typical value for the average water droplet size is about 0.04 μm. This small droplet size not only gives an appearance which is more aesthetically pleasing to the user but also offers several major advantages over the larger droplet-sized systems. These translucent or clear microemulsions tend to be more stable than the larger droplet sized milky macroemulsions, as the water droplets remain in dispersion longer and do not readily undergo macro oil/water phase separation. The small droplet size also appears to negate the need for both corrosion inhibitors and bactericides.
U.S. Pat. No. 3,095,286 (Andress et al) discloses the problem of water accumulation in fuel oil storage tanks, resulting from the “breathing” of storage vessels, presenting a problem of rusting. To inhibit sedimentation, screen clogging and rusting in fuel oil compositions during storage it is disclosed to use a compound selected from a phthalamic acid, a tetrahydrophthalamic acid, a hexahydrophthalamic acid and a nadamic acid and their salts of primary amines having between 4 and 30 carbon atoms per molecule as an addition agent to the fuel oil. There is no disclosure of the addition agents forming water-in-oil microemulsions of the fuel oil.
U.S. Pat. No. 3,346,494 (Robbins et al) discloses the preparation of microemulsions employing a selected combination of three microemulsifiers, specifically a fatty acid, an amino alcohol and an alkyl phenol.
FR-A-2373328 (Grangette et al) discloses the preparation of microemulsions of oil and salt water by employing sulphur containing surfactants.
U.S. Pat. No. 3,876,391 (McCoy et al) discloses a process for preparing clear, stable water-in-petroleum microemulsions, which may contain increased quantities of water-soluble additives. The microemulsions are formed by use of both a gasoline-soluble surfactant and a water-soluble surfactant. The only water-soluble surfactants employed in the worked examples are ethoxylated nonylphenols.
U.S. Pat. No. 4,619,967 (Emerson et al) discloses the use of water-in-oil emulsions for emulsion polymerisation processes.
U.S. Pat. No. 4,744,796 (Hazbun et al) discloses stable water-in-fuel microemulsions employing a cosurfactant combination of tertiary butyl alcohol and at least one amphoteric, anionic, cationic or nonionic surfactant. Cocoamidobetaines are disclosed as possible amphoteric surfactants.
U.S. Pat. No. 4,770,670 (Hazbun et al) discloses stable water-in-fuel microemulsions employing a cosurfactant combination of a phenyl alcohol and at least one amphoteric, anionic, cationic or nonionic surfactant. Cocoamidobetaines are disclosed as possible amphoteric surfactants.
U.S. Pat. No. 4,832,868 (Schmid et al) discloses surfactant mixtures useful in the preparation of oil-in-water emulsions. There is no disclosure of any water-in-oil microemulsion comprising at least 60 wt % oil phase.
U.S. Pat. No. 5,633,220 (Cawiezel) discloses the preparation of a water-in-oil emulsion fracturing fluid including an emulsifying agent sold by ICI under the trademark Hypermer (Hypermer emulsifying agents are not disclosed as being C6-C15 alcohol ethoxylates or mixtures thereof).
Mixtures of C6-C15 alcohol ethoxylates are commercially available surfactants normally sold for use in the preparation of e.g. washing detergents.
WO-A-9818884 discloses water-in-fuel microemulsions, including examples of such emulsions comprising a C8 alcohol ethoxylate, with 6 EO groups, mixed with a polyglyceryl-4-monooleate, and mixtures of C9-C11 alcohol ethoxylates mixed with either polyglyceryl oleates linear alcohols or POE sorbitan alcohols. The presence of the polyglyceryl oleates and POE sorbitan alcohols tend to have detrimental effects on the viscosity properties of the emulsions which, in turn, has a consequential detrimental effect on the lubricity properties of the emulsion.
WO-A-9850139 discloses a water-in-oil microemulsion, including a surfactant mixture comprising a fatty acid amine ethoxylate, a C6-C15 alcohol ethoxylate and optionally a tall oil fatty acid amine. The water-in-oil microemulsion may be an industrial lubricant.
WO-A-0053699 discloses a water-in-oil microemulsion, including emulsifying agents comprising a C6-C15 alcohol ethoxylate, an amine ethoxylate and a polyisobutylsuccininide or sorbitan ester. The water-in-oil microemulsion may be a fuel.
EP-A-1101815 discloses a fuel, particularly for diesel engines, in microemulsion form, comprising a liquid fuel, an emulsifier and an emulsive agent, the emulsive agent having an HLB value higher than 9.
U.S. Pat. No. 6,716,801 discloses a stable, clear water-in-oil microemulsion consisting of from about 5 to 40 wt % aqueous phase and from about 95 to about 60 wt % non-aqueous phase. The microemulsion includes from about 5 to 30 wt % emulsifiers consisting of i) a mixture of C6-C15 alcohol ethoxylates each comprising from 2 to 12 EO groups, 0 to about 25 wt % polyisobutylsuccinimide and/or sorbitan ester, and iii) 0 to about 90 wt % amine ethoxylate. The microemulsion is described to be useful as a fuel and/or lubricant/coolant.
Mixtures of liquid emulsifying agents suitable for use in the preparation of water-in-oil microemulsions are disclosed in WO-A-07083106. Such mixtures, commonly referred to as concentrates, comprise about 0.5 to about 15 wt % fatty (C8-C24)-amido-(C1-C6)alkyl betaine, about 5 to about 99 wt % C6-C15 alcohol ethoxylate comprising from 2 to 12 EO groups or a mixture of such alcohol ethoxylates, preferably the mixture, 0.5 to about 15 wt % (C6-C24)alkyl amine oxide and 0 or upto about 94 wt % other non-ionic emulsifying agent based on the total weight of emulsifying agent in the emulsion.
None of the above prior art references, however, discloses the performance of water-in-oil emulsions at low temperatures.